Corrosion protection pigments have been known for a long time. Used as additives in paints, varnishes, and other coating agents, they are supposed to prevent corrosion of treated metallic surfaces and, like rust converters, favorably eliminate already existing corrosion products. Naturally, they have to be compatible with the coating agent in this regard.
Three characteristic modes of action distinguish corrosion protection pigments.
Pigments producing a physical anticorrosive effect, such as micaceous iron ore, are chemically inert and are referred to as inactive and/or passive pigments whose mode of action is that they considerably extend the diffusion pathways for water, oxygen, and corrosion-enhancing ions and improve the subsurface adhesion of the coat or finishing.
Pigments producing a chemical anticorrosive effect, for example, zinc oxide, possess a certain solubility and are capable of stabilizing or maintaining certain pH values in the coating. Such pigments are referred to as being active. Their mode of action is based on interactions at the transition between pigment and subsurface, pigment and binder, as well as pigment and incorporated foreign ions. Here, redox reactions that create new protective compounds may occur. Due to saponification with the binder and/or neutralization of the acid decomposition products a specific pH value in the coat remains nearly constant. Corrosive ions are eliminated.
Pigments producing electrochemical anticorrosive effects function by passivating actions on the metal surfaces to be protected. These anticorrosive pigments are characterized by whether they are effective in the anodic or cathodic range. Pigments preventing the dissolution of metal by forming a protective layer are designated as being active in the anodic range. Pigments which exhibit high oxidation potential blocking the formation of oxidation products, such as rust in the case of iron, are designated as being active in the cathodic range.
The increasing significance of preventing corrosion and the knowledge of health and environmental risks associated with anticorrosive pigments containing lead and chrome have led to the further development of less toxic anticorrosive pigments on the basis of phosphates, particularly Zn3(PO4)2, or if appropriate, Zn3(PO4)2, in combination with AlPO4, ZnO as well as molybdate-tungstenate and zirconate pigments, metallic zinc, organic pigments and pure barrier pigments such as micaceous iron ore.
The currently used zinc phosphates, however, are classified as toxic to fishes and various proposals for their substitution have already been made.
The application of polymeric phosphates as an inorganic anticorrosive layer has been described in O. Kusnierik, K. Barton, Farbe+Lack 11, 900-901 (1998). Aluminum polyphosphates are to be applied.
A publication series entitled “Studies on anticorrosive properties of aluminium triphosphate pigments” in the Polymers Paint Colour Journal is concerned with the efficacy of aluminum triphosphates as anticorrosive pigments. For example, Nishihara et al. 174, 590-597 (1984) upon comparing zinc chromates describe in Part 1 that alkyd resin coatings have the same or even superior anticorrosive properties. Part 4, J. Nakano et al. 175, 704-706 (1985) describes the effectiveness against the formation of white rust.
Other approaches attempt to enhance efficacies by a partial replacement of zinc, for example, see P. Kalenda et al. in: Macromol. Symp. 187, 397-406 (2002) and EP 1 116 756.
EP 1 109 867 reveals anticorrosive pigments from surface-coated solid state bodies in which Mn3(PO)4 and aluminum oxides and/or aluminum hydroxides are precipitated.
EP 1 029 901 refers to combinations of inorganic anticorrosive pigments in which one or several pigments, selected from phosphate, polyphosphate and metal oxide pigments, are precipitated together with BaSO4.
None of the anticorrosive pigments described completely fulfill all requirements regarding efficacy, processability, health and environmental safety. There is still a need for useful anticorrosive pigments and the task to provide such pigments.
Surprisingly, it was now discovered that aluminum orthophosphate displays a very good property spectrum regarding all three requirements mentioned above.
The three-fold requirements are satisfied by an anticorrosive pigment consisting of aluminum orthophosphate.